This project was undertaken to test the hypothesis that embryonic development and differentiation are mediated by the sequential expression of tissue specific recognition or contact proteins responsible for the cohesion of cells belonging to the same tissue or morphogenetic structure. The test system chosen is the developing sea urchin embryo because these embryos can be taken apart into single cells and reassembled spontaneously into aggregates capable of developing to the final larval state (pluteus). This makes it an ideal assay system for the isolation of membrane components essential for cell/cell contact. In previous work we have been able to render dissociated cells incapable to reaggregate by removing proteins from the surface with 2.5 percent butanol and to restore both reaggregtion and embryonic development by adding back solubilized membrane proteins (Noll et al., 1979). Reaggregation could also be blocked in a species specific manner with monovalent Fabs against membrane components. The proposal has as its goal (1) the purification and characterization of the surface proteins essential for reaggregation and determination of their stage and tissue specificity, (2) elucidation of the mechanism of uptake of the active proteins into the membrane and contact formation with other cells, and (3) identification of the surface receptor tht promotes DNA synthesis when combined with the cognate Fab fragment (Vtorelli et al., 1980).